1. Field of the Invention
Embodiments of the invention relate to a silicon thin film solar cell including a back reflection layer formed of hydrogenated microcrystalline silicon oxide.
2. Description of the Related Art
Solar cells use an infinite energy source, i.e., the sun as an energy source, scarcely produce pollution materials in an electricity generation process, and have a very long life span equal to or longer than 20 years. Furthermore, the solar cells have been particularly spotlighted because of a large ripple effect on the solar related industries. Thus, many countries have fostered the solar cells as a next generation industry.
Most of the solar cells have been manufactured based on a single crystal silicon wafer or a polycrystalline silicon wafer. In addition, thin film solar cells using silicon have been manufactured in lesser quantities.
The solar cells have the problem of the high electricity generation cost compared to other energy sources. Thus, the electricity generation cost of the solar cells has to be greatly reduced so as to meet a future demand for clean energy.
However, because a bulk silicon solar cell manufactured based on the single crystal silicon wafer or the polycrystalline silicon wafer now uses a raw material having a thickness of at least 150 μm, the raw material cost, i.e., the cost of silicon cost makes up most of the production cost of the bulk silicon solar cell. Further, because the supply of the raw material does not meet the rapidly increasing demand, it is difficult to reduce the production cost of the bulk silicon solar cell.
On the other hand, because a thickness of the thin film solar cell is less than 2 μm, an amount of raw material used in the thin film solar cell is much less than an amount of raw material used in the bulk silicon solar cell. Thus, the thin film solar cell is more advantageous than the bulk silicon solar cell in terms of the electricity generation cost, i.e., the production cost. However, an electricity generation performance of the thin film solar cell is one half of an electricity generation performance of the bulk silicon solar cell given the same area.
The efficiency of the solar cell is generally expressed by a magnitude of a power obtained at a light intensity of 100 mW/cm2 in terms of percentage. The efficiency of the bulk silicon solar cell is about 12% to 20%, and the efficiency of the thin film solar cell is about 8% to 9%. In other words, the efficiency of the bulk silicon solar cell is greater than the efficiency of the thin film solar cell.
Accordingly, much effort has been expended to increase the efficiency of the thin film solar cell. As a result, a double junction thin film solar cell and a triple junction thin film solar cell have been developed.
The most basic structure of the thin film solar cell is a single junction structure. A single junction thin film solar cell has a structure in which a photoelectric conversion unit including an intrinsic semiconductor layer corresponding to a light absorption layer, a p-type doped layer, and an n-type doped layer is formed on a substrate. The p-type doped layer and the n-type doped layer are respectively formed on and under the intrinsic semiconductor layer, thereby forming an inner electric field for separating carriers produced by solar light.
The efficiency of the single junction thin film solar cell including the one photoelectric conversion unit is low. Thus, a double junction thin film solar cell including the two photoelectric conversion units and a triple junction thin film solar cell including the three photoelectric conversion units have been developed, so as to increase the efficiency of the thin film solar cell
Each of the double junction thin film solar cell and the triple junction thin film solar cell have the configuration in which a first photoelectric conversion unit first absorbing solar light is formed of a semiconductor material (for example, amorphous silicon) having a wide band gap and absorbs solar light of a short wavelength band, and a second photoelectric conversion unit later absorbing the solar light is formed of a semiconductor material (for example, microcrystalline silicon) having a narrow band gap and absorbs solar light of a long wavelength band. Hence, the efficiency of each of the double junction thin film solar cell and the triple junction thin film solar cell is greater than the efficiency of the single junction thin film solar cell.
The increase in the efficiency of the silicon thin film solar cell requires an increase in a current density flowing in the silicon thin film solar cell. Thus, the silicon thin film solar cell has to be configured so that solar light passing through the intrinsic semiconductor layer is reflected back towards the intrinsic semiconductor layer and then is absorbed in the intrinsic semiconductor layer. As a result, the silicon thin film solar cell includes a back reflection layer for increasing a light absorptance of the intrinsic semiconductor layer, thereby increasing the current density.